Optically clear structural laminate

ABSTRACT

An optically clear structural laminate includes a thermosetting resin, a silane coupling agent and a filler. The laminate has a high weight to strength ratio and is capable of optical transmission over a wide range of temperatures. The laminate has increased tensile strength and is capable of being easily formed into complex shaped components. The structural properties of the laminate make it useful as aircraft canopies and windows.

GOVERNMENT INTERESTS

The invention was made with Government support under a contract awardedby Det 1 AFRL/PKN (−8100), Wright-Patterson AFB OH. The Government hascertain rights in this invention.

BACKGROUND OF THE INVENTION

The present invention generally relates to structural laminates andmethods for producing the same. More specifically, this inventionrelates to optically clear structural laminates incorporating filler,such as fiberglass, and an optically clear resin matrix.

Transparent materials have been widely used in a variety of articles,including glazing materials for buildings and vehicles, laboratoryglassware, packaging, decorative lighting fixtures, safety panels atsports arenas, and architectural panels. Common transparent materialsinclude glass and polymers, such as acrylic and polycarbonate.

Glass may be the most widely used of these materials. Glass is hard,chemically inert in the presence of most other substances, resistant toabrasion, and inexpensive. These qualities make glass useful in manyapplications. However, glass is also heavy, brittle and difficult toshape into complex forms. Standard sheet glass, for example, may havelow tensile properties of about 1,000 psi. These qualities make glassunsuitable for some applications.

Polymers, such as acrylic and polycarbonate, may be lighter in weight,less brittle, and easier to form into complex shapes when compared toglass. Plastics, such as polycarbonate, are more resilient than glass,but they can only produce tensile strengths of about 10,000 psi.Polymers have been used extensively in many industries. For example,poly (methyl methacrylate) has found beneficial use in many products.Unfortunately, polymers may not possess the strength of glass. Materialscomprising a combination of glass and polymer have been used inapplications where neither material alone is desirable.

Laminate materials comprising layers of glass, acrylic or polycarbonateplastics, or combinations thereof bonded together by interlayers of apolymeric bonding material have been described. A glass/plasticwindshield, for example, may comprise a glass face ply laminated toacrylic structural plies by means of polyvinyl butyral (PVB)interlayers. Unfortunately, bond failures, or delaminations, at theinterlayer interfaces have been noted. Causes for bond failure mayinclude mechanical or thermal stress, moisture ingress, and bonddeterioration.

Other materials comprising a combination of glass and polymer have alsobeen disclosed. A composite structure for use as pipes or storage tankshas been described as fiberglass fibers disposed in layers andimpregnated in a resinous binder. To balance and distribute materialstrength in all directions, fibers of one layer are disposed at an anglerelative to the fibers of a second layer. Although containment strengthand resistance to delamination is increased, these materials may not beoptically clear and may not be suitable for many applications.

A transparent material has been disclosed in U.S. Pat. No. 5,733,659.This molded material comprises a pair of thermoplastic films and areinforcing resin composition layer interposed between the films. Thereinforcing resin composition comprises a thermoplastic resin, such asan aromatic polycarbonate resin, a glass filler and polycaprolactone.The glass filler is in the form of beads, flakes, powder or choppedfiber strands and the filler is uniformly blended into the resin. Theamount of glass filler is preferably 5 to 30% (about 2-18% by volume) byweight based on a total weight of the molded product. To produce thistransparent material, the thermoplastic films are fitted on the innersurface of an injection mold and then the reinforcing resin compositionis melt-injected into the injection mold. Although increasing the amountof glass filler increases the strength of the transparent material, itis also said to considerably deteriorate the optical properties.Additionally, a primer coating treatment may be necessary to promote athermal fusion between the thermoplastic film and the reinforcing resincomposition. Further, these processes may not be suitable when usingfabric fillers or unchopped fiber strand fillers.

Other methods of producing transparent composites have been described byDay et al. in “Optically Transparent Composite Development,” FinalTechnical Report (Z10045), McDonnell Aircraft Company, 1992. Althoughcomposite strength increased with increasing filler content, this reportalso noted that optical transmission decreased. The transmission deceasewas said to be due to the large number of interfaces where transmissionlosses occurred. By placing fiber fillers where bending stresses arehighest, the amount of filler required for a given flexural strength wasreduced. Unfortunately, these composites may be transparent only over anarrow temperature range and may not possess the strength needed forsome applications.

A laminate formed by polymerizing a monomer while glass fibers aremaintained immersed within the monomer has been described in U.S. Pat.No. 5,039,566. These materials are said to be useful as aircraftcanopies and aircraft windows. Matching the refractive indices of theglass and polymer minimizes the scattering and reflection of light thatnormally occurs at the glass/polymer interface. Unfortunately, thedescribed processes require several hours or days and the opticalclarity of the resulting material is inadequate for some applications.Further, as the percent glass volume increases, the optical clarity ofthe material decreases. In one sample, the percent glass volume was 14and the optical transmission was 43.3%. In a second sample, the percentglass volume was 33 and the optical transmission was about 20%. Althoughthe first sample had higher optical transmission, it also had a modulusof rupture of only 47,796 lbs./in², while the second sample had amodulus of rupture of about 90,000 lbs./in². Additionally, thesematerials may have acceptable optical clarity only when the material isexposed to a narrow range of temperatures and they may be difficult toform into complex shapes. Further, the manufacturing processes providedmay not be desirable for many applications and may require secondaryinstallation steps.

Some of the disadvantages of the '566 patent were addressed in U.S. Pat.No. 5,665,450. For these composites, four-sided glass ribbons, asopposed to cylindrical fibers, were embedded in a polymer sheet. Theribbons were arranged in such a way that one planar surface of theribbon was parallel to the surface of the polymer sheet. This compositesheet was then laminated onto one or both surfaces of a polymer core. Bypositioning the glass near the surface, the strength of the material wassaid to increase without also increasing glass volume and withoutdecreasing the optical transmission. The glass volume was preferablybetween 1% and 25% of the composite volume. Materials containing ribbonswere found to have a higher transmission over a wider temperature rangethan those containing fibers. Unfortunately, optical transmission overstill wider temperature ranges is needed. Also, processes that requirethe careful arrangement of ribbon surfaces parallel to the surface ofthe polymer sheet may not be preferred when high volume laminateproduction is desired. Further, it was noted that a higher maximumtransmission was observed when using the cylindrical fibers.Additionally, because the composite sheets were laminated ontopre-formed polymer cores, these materials may not be preferred whencomplex component shapes are desired. Also, the production of thesematerials may require high pressure. Although, these materials may haveimproved optical properties when compared to the '566 patent, furtherincreases in laminate strength are needed.

As can be seen, there is a need for improved optically clear structurallaminates. Further, there is a need for laminates having improvedoptical properties without a reduction in percent glass volume.Additionally, there is a need for transparent laminates having decreasedweight and increased structural performance. Also, there is a need forlaminates having increased tensile strength and modulus (increasedstiffness), and increased optical clarity. Further, there is a need foroptically clear laminates capable of being easily formed into complexshapes. Moreover, there is a need for laminates having improved opticalproperties over a range of temperatures without the need for four-sidedribbons.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a laminate capable of opticaltransmission comprises a thermosetting resin; a filler embedded in thethermosetting resin, wherein the refractive index of the filler iswithin about −0.010 of the refractive index of the thermosetting resinfor a wavelength between about 400 nm and about 750 nm; and a couplingagent in contact with the filler.

In another aspect of the present invention, a laminate capable ofoptical transmission comprises an epoxy resin; a filler embedded in theepoxy resin, wherein the volume percent of the filler is between about45% and 65% and the refractive index of the filler is within about 0.010of the refractive index of the epoxy resin for a wavelength betweenabout 400 nm and about 750 nm; and a silane coupling agent in contactwith the said filler.

In still another aspect of the present invention, a structural aircraftlaminate capable of optical transmission comprises a thermosettingresin; a filler embedded in the thermosetting resin, wherein therefractive index of the filler is within about 0.010 of the refractiveindex of the thermosetting resin for a wavelength between about 400 nmand about 750 nm; and a silane coupling agent in contact with thefiller, such that the optical transmission varies by less than about 25%over a temperature range from about −10° F. to about 180° F.

In yet another aspect of the present invention, a laminate capable ofoptical transmission comprises an epoxy resin selected from the groupconsisting of Epotek 301, Epotek 302-3M, and Fiber Optics AB9300; afabric filler embedded in the epoxy resin, wherein the volume percent ofthe fabric filler is between about 45% and about 65% and the refractiveindex of the fabric filler is within about 0.010 of the refractive indexof the epoxy resin for a wavelength between about 400 nm and about 750nm; and a silane coupling agent selected from the group consisting ofA1100, Z6020, and Z6040, said silane coupling agent in contact with thefabric filler,—and the optical transmission varies by less than about25% over a temperature range from about −10° F. to about 180° F.

In a further aspect of the present invention, a method of producing alaminate capable of optical transmission comprises the steps ofproviding a filler coated with a coupling agent; positioning the fillerin a mold; impregnating the filler with a heated thermosetting resin;curing the heated thermosetting resin, such that a laminate is formed;and removing the laminate from the mold.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross sectional diagram of an optically clear structurallaminate according to an embodiment of the present invention;

FIG. 1B is a cross sectional diagram of an optically clear structurallaminate according to another embodiment of the present invention;

FIG. 2 is a flow diagram depicting the steps according to a process ofthe present invention; and

FIG. 3 is a photograph of a fiberglass fabric filler without resin andan optically clear laminate according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplatedmodes of carrying out the invention. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the invention, since the scope of theinvention is best defined by the appended claims.

The present invention generally provides optically clear laminatematerials capable of being processed into shaped components and methodsfor producing the same. The laminates of the present invention may findbeneficial use in many industries including automotive, aviation,architectural, sports, marine, military, civil, space, electronics andmedical. The laminates of the present invention may find particular usein aircraft canopies, light covers, filter lenses, architecturalmaterials, and windows.

The laminates of the present invention can comprise an optically clearpolymer, a fiberglass filler, and a coupling agent. The index ofrefraction of the polymer and the filler may match within about 0.010.Unlike the prior art, the polymer of the present invention can be athermosetting resin and the laminate can be optically clear over a rangeof temperatures, such that objects may be visible when viewed though thelaminate. An “optically clear” laminate is defined herein as a laminatehaving at least about 80% optical transmission in the visible spectrumat 75° F. Moreover, unlike the prior art, reducing the refraction anddistortion caused by the coupling agent may increase the optical clarityof the laminate of the present invention. Improving the opticalproperties of the laminate does not require a reduction in the percentglass volume of the laminate, which is also unlike the prior art. In thepresent invention, heating the thermosetting resin prior to transferringthe resin to a mold, which is unlike the prior art, can increase theoptical properties of the laminate by lowering the viscosity of theresin and providing better wet-out the fiber, thus increasing thestrength of the fiber/resin interface. Thermoplastics are heated beforemolding but generally do have the low viscosity and wettingcharacteristics of the thermosetting resin. Further, unlike the priorart, increasing the temperature range in which the laminate is opticallyclear does not require the use of four-sided ribbons.

FIGS. 1A and 1B are cross sectional diagrams of optically clearstructural laminates 20 according to two embodiments of the presentinvention. A filler 25 can be fiber strands 26 that are woven, as seenin FIG. 1A. Or a filler 25 can be fiber strands 26 that are unwoven, asseen in FIG. 1B. Other useful fillers 25 can include chopped fibers andsolid spheres. The filler 25 can be embedded in a polymer 23. As betterseen in FIG. 1B, the filler 25 may be coated with a coupling agent 27.

Polymers 23 useful in the present invention may include thermosettingresins. Thermoplastics may be softened and reformed into differentshapes by heating, whereas thermosetting resins may not. As temperatureincreases, laminates comprising thermoplastics may weaken and warp. Thestrength of thermoplastic laminates may be more dependent on temperaturewhen compared to the strength of thermosetting resin laminates. Thethermosetting resins may have a three-dimensional molecular structurethat provides a crosslinking feature as opposed to the linear molecularstructure commonly found in thermoplastics. Some thermoplastics, such aspoly (methyl methacrylate), do have three-dimensional molecularstructures; however, dipole-dipole forces as opposed to covalent bondsin thermosetting resins may hold their molecules together. Due to theirmolecular structure, materials comprising thermoplastics may have a morepronounced decrease in modulus of elasticity than materials comprisingthermosetting resins. Examples of thermosetting resin advantages mayinclude greater strength and stiffness, relatively low cost comparedwith high performance thermoplastics, high long-term heat resistance andhigh heat distortion temperatures, excellent dimensional stability, andlow predictable mould shrinkage which permits thick and thin sectionstogether in one part and prevents warping and sink marks. Mostthermosets may be harder than thermoplastics and therefore more scratchand abrasion resistant. Thermosets may have low flammabilitycharacteristics. Thermosets do no melt. The low viscosity nature of theselected thermosets may wet out the fiber better than a thermoplasticresin can for a high volume percent (45-65%) loading of fiber within amold. Lower processing temperatures and pressures may be used withthermosets.

Known processes for producing transparent laminates may comprise forminga filler/polymer pre-preg layer and hot-pressing the pre-preg layer ontoa polymer sheet. These processes may not be useful when the polymer is athermosetting polymer because the hot-pressing must be at a temperatureat or above the glass transition temperature of the polymer and theprocesses require a polymer flow sufficient to consolidate the pre-preglayers but insufficient to cause the fillers to lose their alignment.Misalignment may not be a problem with woven materials. Polymer flowoccurs when the polymer molecules move in relation to one another. Theheat or energy required to break the covalent bonds holding thethermosetting polymer molecules together may also break the bondsholding the atoms of the molecule together. Processes that result in thebond breakage within the polymer molecule may not be desirable.

Useful polymers 23 may include phenol-formaldehyde, urea-formaldehyde,melamine-formaldehyde,—polyester resins, and epoxy resins. Melamineresins may be resistant to the influence of light and useful withinorganic fillers. Polyester resins and epoxy resins may be colorlessand useful with inorganic fillers. Epoxy resins may have better chemicalresistance than polyester resins. Epoxy resin laminates may have greatertensile, flexural and impact strength than—polyester resin laminates. Onthe other hand, phenol resins may turn brown under the influence oflight. Urea resins may be colorless and resistant to light; however,they may have a low thermal stability, and they may not be suitable foruse with inorganic fillers.

Preferred polymers 23 of the present invention include epoxy resins.Epoxy resins may be formed by the base-catalyzed reaction between anepoxide, such as epichlorohydrin, and a polyhydroxy compound, such asbisphenol A. This reaction may yield a prepolymer by an initialbase-catalyzed ring cleavage of the epoxide ring by the hydroxyl groups.The resulting product may contain both terminal epoxy groups and pendanthydroxyl groups. The addition of reagents, such as amines, may thenresult in the crosslinking of the prepolymer. Epoxy resins may becharacterized more by ring cleavage and condensation than by simplering-opening polymerization. Useful epoxy resins may include bisphenol—Adiglycidyl ester, bisphenol glycidyl ether, novolac resin glycidyl etherand aliphatic polyepoxide. Preferred epoxy resins include The BisphenolA and F family. Bisphenol F may be preferred for higher temperatureapplications. More preferred thermosetting resins include Epotek 301,Epotek 302-3M, and Fiber Optics AB9300. Epotek 301 and Epotek 302-3M areavailable from Epoxy Technology in Billerica, Mass. AngstromBond AB9300epoxy resin is available from Fiber Optics Center in New Bedford, Mass.

Hardening agents, such as thiocol rubber and polyamide hardener, may beincluded in the polymer resins of the present invention to solidify, orcure, the resins. The thermosetting resin may also be cured without theaddition of a hardening agent. Useful hardening agents, also referred toas curing agents, may include aromatic polyamines, polyamides, aliphaticpolyamines, polyacids and polyanhydrides. Preferred hardening agentsinclude the Polyamine family. Solidifying, or curing, an epoxy resin maybe at room temperature or higher temperatures. Preferred curingtemperatures may be between about 75° F. and about 350° F. A morepreferred curing temperature may be between about 250° F. and about 300°F. The curing rate may be adjusted by the choice of hardening agent andcuring temperature.

According to the present invention, fillers 25, such as fibers andfabric, may be added to the polymer 23. A useful volume percent offiller 25 may be between about 45% and about 65% of the volume of thelaminate. Fabric fillers may be woven or unwoven. Other filler shapes,such as chopped fibers and solid spheres, are known in the art, and maybe useful in the present invention. Methods for producing these fillersare also known in the art. Methods for producing useful glass fillersare described in U.S. Pat. No. 5,665,450 and which is hereinincorporated by reference.

The fillers 25 may carry more of an externally provided load than thepolymer 23. The polymer 23 may serve to transfer stresses from anexternal source, such as an impact force, to the fillers 25 and toprotect the fillers 25 against damage. For example, the polymer 23 maybe more resilient than the filler 25 and may cover the filler 25, suchthat abrasions and breakage of the filler 25 caused by external sourceimpacts may be prevented.

The structural properties of the laminate 20 may depend on theorientation of the filler 25 within the laminate 20. For example,fillers 25, such as fiber strands 26, may be disposed in aunidirectional or cross-ply manner within the laminate 20 such that thetensile strength of unidirectional laminates and quasi-isotropiclaminates may be increased or orientated in desired locations within thelaminate. The altering of filler orientation to alter the structuralproperties of the laminate may be useful in some applications of thepresent invention. For example, when the laminate 20 is an automobilewindshield, fiber strands 26 disposed cross-ply may be preferred tofiber strands 26 disposed unidirectional.

Incomplete polymer 23 penetration into a filler 25, such as a fiberglassfabric 24 shown in FIG. 3, may reduce optical clarity. Incompletepolymer penetration is polymer penetration that results in a laminate 20having at least a portion of filler 25 exposed to air. Because thepolymer 23 may not completely penetrate fabric fillers thicker thanabout 0.020″, thinner fabrics may be preferred. Useful fillers 25 may befabrics between about 0.002″ and about 0.020″ in thickness. Preferredfabric fillers may be between about 0.002″ and about 0.010″ inthickness. One useful fabric may be 108 style fabric available from theHexcel Corporation in California. This fiberglass fabric may be 0.0025″thick. Other types of fabric that may be useful are also available fromthe Hexcel Corporation. These include 120 and 7781 style fabric withthicknesses of 0.005″ and 0.010″, respectively. Increasing the volumepercent of the filler 25 may increase the structural properties, such asrigidity, of the laminate 20. A useful volume percent of filler may bebetween about 45% and about 65%. A preferred volume percent of fillermay be between about 60% and about 62%.

Useful fillers 25 may include glass and quartz. Glass fillers may becomposed of mainly silicon dioxide materials. Useful glasses mayinclude, but are not limited to, type A glass, type E glass, type Cglass, type D glass, types S glass and type R glass. Type A glass may bean alkali glass also referred to as soda lime glass and may commonly beused in windows and bottles. Type E glass may be a boroaluminosilicateglass and may commonly be used in reinforced plastics. Type C glass maybe a calcium aluminosilicate glass and type D glass may be a lowerdensity glass. Type R and S glasses may be high-strength glasses. Eachglass type has different volume fractions of silicon dioxide with otheradditional various oxide components. The silicon dioxide percent canvary from 50 to 75%, giving each type of glass a different index ofrefraction value. Even within the same type of glass, such as E-glass,the index refraction of the base material can vary from vendor tovendor, so it is important to match the index of refraction of the resinwith a particular manufacturer.

Preferred glass fillers include optical glasses. Optical glasses mayhave higher transmission than normal window glasses. Optical glasses,such as flint glasses and crown glasses, may be available from ChanceBros and Co. Ltd in England, Parra Mantois et Cie in France, Schott undGenossen in Germany, Bausch and Lomb Optical Co. in the U.S.A., andCorning Glassworks in the U.S.A. Catalogues of optical glasses may givethe refractive indices at various wavelengths corresponding to certainspectral lines.

The refractive index of the polymer 23 may be matched to the refractiveindex of the filler 25. A preferred difference in the refractive indicesat about 72° F. may be less than about 0.010 for a wavelength betweenabout 400 nm and about 750 nm. The refractive index of a material may bea function of the temperature of the material. The amount of change inrefractive index due to change in temperature may be a function of thecomposition of the material. For example, two different materials mayhave the same refractive index at one temperature and differentrefractive indices at another temperature. For a preferred laminate 20,the filler 25 and polymer 23 may be matched in both index of refractionvs. wavelength and in temperature coefficients of refractive index. Thewavelength of maximum transmission through a material may also be afunction of temperature. For example, the wavelength of maximumtransmission may be 800 nm at one temperature and 400 nm at anothertemperature. The refractive index of the polymer 23 may be matched towithin about 0.010 of the refractive index of the filler 25 over a rangeof temperatures. The difference between the refractive index of thefiller 25 and the refractive index of the polymer 23 may be betweenabout 0.001 and about 0.010. A useful range of temperatures may be about−65° F. to 350° F. A preferred range of temperatures may be fromabout—−40 F° F to about 250° F. A more preferred range of temperaturesmay be from about—−10° F. to about 180° F. The optical transmissionwithin the preferred temperature range may vary by less than about 25%.

The properties of the laminate 20 may also be tailored to providethermochromic characteristics for creating a product that can changecolors with altering temperature. For example, to produce a laminate 20that changes color with changing temperature, the refractive indices ofthe polymer 23 and filler 25 may be matched closely within 0.005 toproduce a clear laminate at room temperature, however if the resin andfiller have different temperature coefficients of refractive index, thiscan produce a laminate that changes color in either the hot or coldcondition. As an example, this type of laminate could be used as avisual tool in determining temperature gradients in thermalapplications.

A coupling agent 27, such as a silane, may be applied to the fillers 25to enhance bonding between the filler 25 and the polymer 23. Thisenhanced bonding may improve the structural properties, such as tensilestrength and (modulus) rigidity, and optical properties, such astransmission, of the laminate 20. This may be because air pockets may beless likely to form around the fillers 25. These air pockets may reducestress transference to the fillers 25 and decrease optical clarity ofthe laminate 20. Silanes may provide superior adhesion between thefiberglass and resin matrix. The silane also may improve heat andmoisture resistance, mechanical properties, weatherability, and solventresistance of the laminate 20. Additionally, the silane may improvewetting of the filler 25. Useful coupling agents 27 may include silane.

Silanes, such as methacryloxypropyltrimethoxy-silane andvinyltriethoxysilane, may comprise two functionalities bound to the samesilicon atom. One functionality may be a nonhydrolyzable organic radicalcapable of interacting with polymers. The other functionality may be ahydrolyzable group. The hydrolyzate product, silanol, may be capable ofreacting with inorganic substrates, such as glass. Preferred couplingagents 27 may include Silquest A-1100(gamma-aminopropyltriethoxysilane), Dow Corning Z6020(Aminorthylaminopropyltrimethoxysilane), and Dow Corning Z6040(3-glycidoxypropyltrimethoxysilane).

The coupling agents 27, such as silane, may also cause light refractionand visual distortion in the laminate 20. Useful coupling agents 27 maydepend on the composition of the polymer 23 and the composition of thefiller 25. The selection of the coupling agent 27 may be based not onlyon its bonding properties within the laminate 20 but also on itsdistortion and refractive properties within the laminate 20. Forexample, when using Epotek 301 epoxy resin and E-glass, a coupling agent27 comprising 3-glycidoxypropyltrimethoxysilane may be useful. When analternate coupling agent, such as Aminorthylaminopropyltrimethoxysilane,is selected for this laminate 20, the coupling agent 27 may have aboutequivalent bonding properties; however, the optical properties of thelaminate 20 may decrease due to a wider differential in refraction anddistortion by the difference in index of refraction between the silane(1.43) and the resin/fiberglass (1.54) materials.

The coupling agent 27 may be applied before and/or after the fibersstrands 26 are formed into a fabric. The coupling agent 27 may beapplied to the fillers 25 during or after the manufacture of the filler25. Methods for applying a coupling agent 27 to a filler 25 are known inthe art. Useful methods may include dipping, extrusion, casting,reaction injection molding, painting, washing, spraying, and others. Thecoupling agent 27 may be applied to the filler 25 such that a filler 25coated with coupling agent 27 is produced. A useful thickness of theapplied coupling agent 27 may be between about 50 angstroms and about500 angstroms. A preferred thickness may be between about 50 angstromsand about 100 angstroms (5-10 monolayers). Since it is very difficult tomeasure the silane thickness, manufacturers apply the coating thicknessof silane to fiberglass as a function of percent solids of silane in asolution. The preferred weight percent of silane in a solution isbetween 0.07% and 0.30% per gram of fiber based on a fiberglass surfacearea of m²/gram. A more preferred weight percent of silane in a solutionis 0.07% to 0.10%. When a hazy finish is noticeable on the fiberglasssurface it means too much silane has been applied. Due to the differencein refractive index between the fiberglass (1.54) and silane (1.43), aminimal coating is desirable to minimize the light scattering effectsbetween the resin and fiberglass.

The laminate 20 of the present invention may be optically clear over arange of temperatures. They may also have increased unidirectional andquasi-isotropic tensile strength. The tensile strength of theunidirectional laminates of the present invention may be between about50 kpsi and about 85 kpsi. Preferred unidirectional laminates may have atensile strength of at least about 50 kpsi. The tensile strength of thequasi-isotropic laminates of the present invention may be between about30 kpsi and about 55 kpsi. Preferred quasi-isotropic laminates may havea tensile strength of at least about 30 kpsi. Laminates 20 of thepresent invention may have a thickness of about 0.002″ to about 0.150″.Preferred thicknesses are between about 0.005″ and about 0.100″. Morepreferred thicknesses are between about 0.010″ and about 0.060″.

The steps according to a process of the present invention are depictedin FIG. 2. The process may comprise a step 30 of providing a filler 25coated with a coupling agent 27 a step 31 of positioning the filler 25in a mold, a step 32 of impregnating the filler 25 with a heated polymer23, a step 33 of curing the heated polymer 23 such that a laminate 20 isformed, and a step 34 of removing the laminate 20 from the mold.

The filler 25 coated with a coupling agent 27 of step 30 may comprise afiberglass fabric coated with a silane. This fabric may then be placedin a mold. Molds for use with thermosetting resins may be divided intotwo groups—compression molds and resin transfer molds. With compressionmolds, the polymer 23 may be placed in the open mold. A product may beshaped by pressure build up as the mold is closed. With resin transfermolds, the polymer 23 may be placed in a chamber outside the closedmold. There may be a gate connecting the chamber to the mold. Thepolymer 23 may be forced, as by a hydraulic plunger, air pressure, avacuum, or by a combination of all three, through the gate and into theclosed mold. Resin transfer molds may be preferred for shaping productswith delicate or thin areas. This may be because the areas of the moldcorresponding to the delicate or thin areas of the product mayexperience less force than when using a compression molding system.Molds used in the present invention may comprise resin transfer moldsand may be used to form laminates having complex shapes, such asaircraft canopies, curved windows, aircraft windshields, gauge covers,and light covers. Useful Resin Transfer Mold (RTM) processes aredescribed in U.S. Pat. No. 5,730,922, and which is herein incorporatedby reference. Releasing layers, such as Teflon film, used in RTMprocesses are also known in the art and may be useful in the presentinvention. Functional layers, such as anti-fogging, radiation-shielding,hard coat, anti-reflective and anti-static, are also known in the artand may be useful in the present invention.

A step 32 of impregnating the filler 25 may comprise a vacuum. As isknown in the art, polymer 23 may be transferred through a gate into amold during RTM processes. The polymer 23 may be transferred by anysuitable means, such as hydraulic or mechanical plunger. For the presentinvention, the preferred means of transferring the polymer 23 may be byvacuum. A vacuum may be created in the mold. This vacuum may then drawthe polymer 23 through a gate and into the mold such that the filler 25becomes impregnated with the polymer 23 and entrapped air within thefiber bundles is removed.

The polymer 23 may be heated and degassed prior to being transferredinto the mold. A heated polymer 23 may have a lower viscosity than anunheated polymer 23. A lower viscosity polymer 23 may be capable ofincreased wetting and penetration of the filler 25. The increasedwetting and penetration may improve the structural and opticalproperties of the laminate 20. A useful temperature to which the polymer23 is heated may vary depending on the composition of the polymer 23 andthe dimensions of the filler 25. The polymers 23 of the presentinvention may be heated to a temperature high enough to lower theviscosity of the polymer and low enough to prevent the polymer fromexotherming. For example, when using Fiber Optics AB9300 epoxy resin and35 plies of 108 style fiberglass cloth filler, a temperature of about150° F. may be preferred. Prior to impregnating the filler 25, thepolymers 23 of the present invention may be heated to a temperaturebetween about 140° F. and about 160° F. The polymers 23 may also bedegassed prior to impregnating the filler 25. Degassing techniques areknown in the art and may comprise a vacuum system utilizing a minimum of28 inches of mercury. The degassing may reduce the volume of air in thepolymer 23 such that the mechanical and optical properties of thelaminate 20 are improved.

The step 33 of curing the heated polymer 23 such that a laminate 20 isformed and the step 34 of removing the laminate 20 from the mold areknown in the art. The step 33 of curing may include increasing thetemperature or pressure of the polymer 23. When the laminate 20comprises one or more functional layers, the functional layer may beco-cured with the polymer 23. For example, an ultraviolet protectionfilm may be placed as the outside layer in the laminate and co-curedwith epoxy resin. Preferred layers include diffusion, anti-reflection,and ultraviolet protection layers and they may be co-cured with thepolymer 23. Any curing and removing methods known in the art may beuseful in the present invention.

EXAMPLE 1

An optically clear structural laminate was produced according to anembodiment of the present invention. A fiberglass fabric 24, as seen inFIG. 3, was purchased from the Hexcel Corporation in California. Thefiberglass used was 108 style fabric 24 and was 0.0025″ thick. Thefabric 24 had a refractive index of 1.550. The coupling agent wasalready applied to the fabric by the vendor. The coated fabric was thenplaced in the RTM mold. A 100 gram sample of AngstromBond AB9300 epoxyresin available from Fiber Optics Center in New Bedford, Mass. wasdegassed for 10 minutes in a bell jar using 28 inches of mercury andheated to a temperature of 150° F. for 20 minutes. The polymer 23 had arefractive index of 1.548 at 589 nm. The heated polymer was thentransferred to the mold by vacuum. The polymer impregnated fabric wasthen cured for 16 hours at 150° F. and postcured at 250° F. for 3 hourssuch that a laminate 20 was produced.

Other laminates 20 of the present invention may comprise resins such asEpotek 301 resin from Epoxy Technology in Billerica, Mass. and may beprocessed at room temperature and later postcured at highertemperatures. The laminate 20 in FIG. 3 was processed using the RTMmethod at room temperature using Epotek 301 resin. The laminate 20 wasoptically clear over a temperature range from −10° F. to 180° F. and hada thickness of 0.065″. The fiberglass/301 resin laminates may provide anoptical transmission of about 96%, 91%, and 86% for laminate thicknessesof 0.005″, 0.030″, and 0.065″ respectively. For comparison, an industrystandard Fiberglass or quartz laminate with epoxy at 0.065″ has lessthan 10% transmission.

As can be appreciated by those skilled in the art, the present inventionprovides improved optically clear structural laminates and methods fortheir production. Also provided are transparent laminates that areeasily formed into complex shapes such as aircraft canopies, aircraftwindshields, gauge covers, and light covers. Further, the presentinvention provides structural laminates with optical clarity over a widerange of temperatures, such as a range of about −10° F. to about 180° F.As can be appreciated by those skilled in the art, laminates areprovided that may be co-cured with other materials, such asanti-reflective layers and radiation-shielding layers, therebyeliminating the costs associated with secondary installation steps insome applications. Outside mold surfaces may be modified or textured toprovide diffusion characteristics in the laminate surface. Additionally,a laminate comprising polymer and glass fiber is provided, wherein thetemperature range for optical transmission is increased without the needfor glass ribbons. Moreover, the modulus of this structural laminate ismuch greater than the thermoplastics with chopped fiberglass strands.Also provided is an optically clear structural laminate that has atensile strength of at least about 30 kpsi for an isotropic laminate and50 kpsi for a unidirectional laminate.

It should be understood, of course, that the foregoing relates topreferred embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

1-28. (canceled)
 29. A method of producing a laminate capable of opticaltransmission comprising the steps of: providing a filler coated with acoupling agent; positioning said filler in a mold; impregnating saidfiller with a heated thermosetting resin; curing said heatedthermosetting resin, such that said laminate is formed; and removingsaid laminate from said mold.
 30. The method of claim 29, wherein saidheated thermosetting resin is a heated epoxy resin.
 31. The method ofclaim 30, wherein said heated epoxy resin is selected from the groupconsisting of Bisphenol A and Bisphenol F families.
 32. (canceled) 33.The method of claim 29, wherein said laminate is a unidirectionallaminate having a tensile strength of at least about 50 kpsi.
 34. Themethod of claim 29, wherein a refractive index of said filler is withinabout 0.010 of a refractive index of said thermosetting resin for awavelength between about 400 nm and about 750 nm.
 35. The method ofclaim 29, wherein said filler is a fabric filler having a thicknessbetween about 0.002″ and about 0.010″.
 36. The method of claim 29,wherein said optical transmission varies by less than about 25% over atemperature range from about −10° F. to about 180° F.
 37. The method ofclaim 29, wherein said heated thermosetting resin is at a temperaturebetween about 140° F. and about 160° F.
 38. The method of claim 29,wherein said laminate is selected from the group consisting of aircraftcanopy, aircraft windshield, gauge cover, and light cover.
 39. Themethod of claim 29, wherein said curing further comprises co-curing atleast one functional layer.
 40. The method of claim 39, wherein saidfunctional layer is selected from the group consisting of diffusion,anti-reflection, and ultraviolet protection layers.
 41. A method offorming a structural window, comprising the operations of: providing anepoxy resin having an index of refraction; coating a filler, said fillerhaving an index of refraction, with a coupling agent, said couplingagent having a coupling component for covalently bonding said couplingagent with said epoxy resin; reinforcing said epoxy resin with asufficient amount of said coated filler to provide structural strengthto said epoxy resin, wherein said coupling agent mixes with said epoxyresin, and wherein: the index of refraction of the resin and the indexof refraction of filler are matched so that the window transmits atleast about 80% of incident light in the visible spectrum at 75 degreesF.
 42. The method of claim 41 wherein said coupling agent includes analkoxysilane.
 43. The method of claim 41 wherein said coupling componentof said coupling agent includes at least one of an amine or an epoxide.44. The method of claim 43 wherein said coupling agent includes at leastone coupling component selected from the group consisting of:3-glycidoxyalkyl, aminoalkyl, and aminoethylaminoalkyl.
 45. The methodof claim 41 wherein said reinforcing operation comprises providingfiller in at least 45% of the volume of the structural window.
 46. Themethod of claim 41 wherein said reinforcing operation comprisesproviding said filler including at least one ply of fabric.
 47. Themethod of claim 46 wherein said fabric is woven.
 48. A method of forminga structural laminate window, comprising the operations of: providing anepoxy resin having an index of refraction; coating a filler, said fillerhaving an index of refraction, with a coupling agent, said couplingagent having a coupling component for covalently bonding said couplingagent with said epoxy resin; impregnating a plurality of layers of saidfiller with said epoxy resin, wherein said coupling agent mixes withsaid epoxy resin; stacking said plurality of layers to form a laminate;curing said laminate to form the structural laminate window wherein: theindex of refraction of the resin and the index of refraction of fillerare matched so that the window transmits at least about 80% of incidentlight in the visible spectrum at 75 degrees F.
 49. The method of claim48 wherein said impregnated layers are fiberglass fabric coated with asilane.
 50. The method of claim 48 wherein said impregnating operationincludes pre-heating said epoxy resin in a temperature rangesufficiently high to lower the viscosity of said epoxy resin andsufficiently low to prevent said epoxy resin from exotherming.
 51. Themethod of claim 48 wherein said impregnating operation is accomplishedusing a resin transfer mold process.
 52. The method of claim 48 whereinsaid curing operation is performed in an autoclave at a temperatureexceeding 150° F.